In a conventional example of a continuous casting mold used for steel, copper or a copper alloy that has a superior thermal conductivity has been used as a mold base material, and the inner face thereof is constantly came in contact with high-temperature molten steel. This is same situation in the case of a continuous casting mold made of brass. The inner wall of the casting mold to come in contact with the high-temperature molten steel receives severe damages, there has been a conventional trend that in an attempt to achieve abrasion resistance, heat resistance, etc., the surface of the casting mold inner face is coated with a coating material to prolong the service life of the mold, and at present, the trend has been unchanged. At the time when the continuous casting method was first introduced, starting with chromium plating, nickel plating or the like was then utilized, and in order to further improve the durability, coating materials, such as nickel-phosphorous alloys, nickel-iron alloys, cobalt-nickel alloys and nickel-chromium self-melting alloys formed through thermal spraying, have been continuously proposed. At present, these coating are appropriately combined and designed to form a best-suited coating structure for each mold for continuous casting machines and the resulting coating is used. Among continuous casting machines, there is an operation case in which scraps derived from, for example, zinc plated steel plates, are used during its molten steel producing process, and this trend is particularly strong upon casting in an electric furnace. When molten steel containing zinc as impurities is subjected to a casting process, the inner wall of a casting mold, in particular, a molten-metal-contact part (generally, referred to as meniscus position), is susceptible to erosion in the copper material and protective coating due to molten zinc, and the molten zinc is dispersed and invades through cracks in the protective coating to sometimes form alloys. Moreover, anchoring of zinc onto the surface of the mold frequently takes place, causing a further reduction in thermal conductivity and the resulting temperature increase in the molten metal contact part and reduction in the thermal fatigue resistant property of the copper base material, with the result that heat cracks tend to occur to damage the base material of the mold.
Patent Document 1 (Japanese Examined Patent Publication No. 04-2337) has proposed a technique for preventing adverse effects due to molten zinc, wherein cobalt or a nickel alloy containing 10 mass % or more of cobalt is utilized as a zinc dispersion preventive coating. As shown in Patent Document 1, a cobalt-based metal has the advantage that with respect to zinc in a molten state, the erosion rate is made slower in comparison with nickel-based metals; however, since cobalt is contained as a constituent component, the adhesive strength to zinc is comparatively high, resulting in the necessity of constantly removing adhered matters and the subsequent time-consuming, complex maintenance processes during operation are pointed out as a problem.
In recent years, under circumstances in which molten steel containing zinc as impurities is almost generally processed and there has been a strong demand for introducing an electromagnetic stirring method into a continuous casting machine so as to improve the quality of cast blocks. In accordance with this trend, the application of a mold base material having high permeability, or a low thermal conductivity, that is adopted so as to improve the stirring effect for molten steel, and an increased casting rate that is adopted so as to improve the productivity cause a higher temperature on the inner wall of the casting mold, in particular, on a molten metal contact face, with the result that the thermal fatigue resistant property further deteriorates. In this manner, adverse effects from the high-temperature meniscus position and an increase in the amount of molten steel cast containing zinc as impurities constantly cause occurrence of heat cracks in an earlier stage and an increase in the mold reproduction cycle as well as an increase in the amount of disposed molds.
Patent Document 2 (Japanese Patent No. 3004870) has described that by applying nickel plating to the surface of a base material made of copper or a copper alloy, and then by forming two or more chromium layers having a purity of 99% or more to provide a thickness of 25 μm or less, it becomes possible to delay erosion due to zinc from molten steel, and consequently to prolong the service life of the mold and the life time up to disposal thereof. Patent Document 3 (Japanese Patent Application Laid-Open No. 2004-237315) has proposed a method wherein two or more low-hardness chromium plated layers having a Vickers hardness of 600 or less are formed in a range with 300 mm from the upper end of the mold, and a cobalt alloy containing a predetermined amount of cobalt or nickel is placed as the under layer so that it becomes possible to prevent damages to the meniscus position of the casting mold.
Moreover, Patent Document 4 (Japanese Patent Application Laid-Open No. 10-156490) has proposed a method wherein at least a meniscus position of a mold that comes directly in contact with molten steel is coated with chromium plating having a single or two compression stress layers so that erosion due to zinc from the molten steel is prevented. In any of these techniques proposed by Patent Documents 2 to 4, chromium or chromium-based metal plating is applied so that the characteristic that the low affinity to zinc of chromium is utilized. In other words, these techniques utilize the electroplating that is easily applicable in the industrial point of view so that multiple chromium plating layers are prepared to reduce the possibility of cracks reaching the base material, or the number of cracks is reduced by making the hardness lower, or the cracks are prevented from expanding by applying a compression stress to the chromium plating layer. However, even in the case when the plated coating originally having cracks is made to have a low hardness to reduce cracks or when multiple layers are formed to avoid cracks from reaching the base material, since chromium itself has a small thermal expansion coefficient and is a metal having a low extension, in the event of an invasion of molten zinc, it is not possible to prevent the under metal layer from being eroded, even though the service life can be prolonged.
Patent Document 5 (Japanese Patent Application Laid-Open No. 2004-25244) has disclosed a method in which, in a vacuum chamber, an injection layer is formed on a specific portion of a copper mold material by directly ion-injecting a metal which has a high melting point and low affinity to zinc, such as chromium, molybdenum and tungsten, so that invasion of molten zinc is prevented. However, since a casting mold, which is a large-size structural object, needs to be housed in the vacuum chamber, the device inevitably becomes bulky, resulting in a problem of high costs.
Moreover, Patent Document 6 (Japanese Patent Application Laid-Open No. 08-132186) has disclosed a method in which a coating of a silicon polymer or a silicon compound is formed on the surface of a chromium plated inner wall face of a mold, and baked thereon at a temperature of 500° C. or less. In this method, the silicon compound is permeated into the cracks of the chromium plating layer and plugged therein so as to prevent zinc from invading therein. This method also requires a large-size furnace in the same manner as Patent Document 5, and causes problem with a permeating function of the silicon compound into cracks, resulting in failure of practical use.
Moreover, in Patent Document 7 (Japanese Patent Application Laid-Open No. 07-303942), a mold has been proposed in which: first, a first plated layer made of an alloy of cobalt or iron and phosphorous or a cobalt-iron-phosphorous alloy is formed on the surface of a mold base material, and a second plated layer made of a cobalt single layer is then formed, with a chromium plated layer being placed as the outermost layer. This mold attempts to prevent zinc invading from the chromium plating by using a low zinc erosion property of cobalt or a cobalt alloy; however, this method has failed to exert a satisfactory life-prolonging effect.
Here, Patent Document 8 (Japanese Patent Application Laid-Open No. 09-52152) has proposed a molten zinc erosion preventive method for a continuous casting mold of brass. In this method, a layer, which contains molybdenum, vanadium or molybdenum and vanadium in 60 mass % or more, with metals, such as copper or iron, cobalt and nickel, and an alloy of these being contained therein as the other components, is formed on a contact face of a mold made of copper or a copper alloy to molten metal with a thickness of 10 μm or more, by using a plating method, a thermal spraying method, a sputtering method, an ion plating method or a CVD method so that transfer of zinc in brass into flux and the resulting anchoring of zinc onto the mold surface are prevented. However, the formation of a plated coating containing 60 mass % of molybdenum and vanadium by the use of an electroplating method is impossible in principle, and the method is actually limited to the thermal spraying method, sputtering method, ion plating method, CVD method or the like, resulting in a problem with adhesion and requiring a special chamber; consequently, it is almost impossible to utilize this method in the industrial field.
Moreover, molten zinc or its alloy is plated on the steel plate to add rust preventive capability thereof, and with respect to countermeasures to molten zinc which adversely affects sink rolls and support rolls in this plating line, a thermal sprayed coating has been used in many cases. The reason for this is because the thermal spraying method has an advantage in that a composite coating made from various components such as carbides, nitrides and borates can be formed and because conventionally, the electroplating method has failed to produce a material that can withstand zinc in a molten state. Patent Documents 9 (Japanese Examined Patent Publication No. 07-13292) and Patent Document 10 (Japanese Patent No. 2986590) show examples in which the thermal sprayed coating is adopted.